The present invention relates to special effects generators in video systems. A special effects generator produces a scene which is a combination of two or more video signals. The combination may be a simple additive mix or a more complicated function which consists of combination mixes, effects, and keys. The special effects may for example include horizontal split wipe effects, wherein a first signal input is displayed on the screen during the first half of each line and a second input during the second half of each line. Keying is defined as the blanking of the picture in response to a keying signal. Numerous other special effects can of course be produced and are to be included in the scope of this invention. In presently known systems, the video signals to be mixed are routed first through a mixing unit which performs the mix function; the output of the mixing unit is then applied to the input of the effects unit where the particular special effects are generated. Additional mixing and further effects may be required, causing the video signals to be passed through a number of units before the final special effect has been achieved.
The conventional units thus have a number of associated problems. First, each mixing or effects element creates a time delay. This time delay must be equalized for all signals entering a particular unit. However, the delay lines which are utilized to create an equivalent delay have frequency characteristics for which compensation must then be added. Further, the delay lines must be properly terminated, thereby requiring a driving circuit which in turn must have frequency compensation. Another difficulty is that for different applications, delay lines may have to be switched in and out, which requires additional logic. Still another difficulty is that the correct DC level and amplitude have to be maintained throughout the rerouting of the signals. Unless great care is taken, the levels tend to drift in operation and the difference in level becomes visible as an undesired flash in the picture. Still another difficulty is that a great deal of space is required for the equipment and in particular for the delay lines and that the adjustment of the delay lines is a very time consuming process. Finally, the conventional systems must be periodically realigned, which is a lengthly process and requires relatively skilled personnel.